A physical random access channel (PRACH) is a contention-based channel that has been implemented in various wireless communication systems for initial uplink (UL) transmission. In general, a particular PRACH implementation is dependent on the technology employed by an associated wireless communication system. For example, depending on the wireless communication system, a PRACH can be used to access a network, request resources, carry control information, adjust a time offset of a UL, and/or adjust transmitted power. As the PRACH is a common channel, the PRACH may experience collisions when different user equipment (UE) attempts to simultaneously utilize the PRACH. In order to help prevent a collision on a message of interest, a system may employ a preamble (which is a short signal that is typically sent prior to a transmission of an associated message) for a PRACH access. That is, a PRACH access may include a transmission of a preamble (that is selected from a set of preambles) and a subsequent transmission of an associated message. In a long-term evolution (LTE) compliant wireless communication system, a PRACH only includes a preamble.
In at least one wireless communication system, a UE may persist in sending a preamble (at least for a predetermined number of times) until the UE receives an acquisition indicator (AI) or a random access response message from a serving base station (BS) that indicates the BS correctly detected the preamble. When a positive AI or a positive random access response message is received by a UE, a subsequent transmission of an associated message is contention free, except where multiple UEs have transmitted the same PRACH signal substantially simultaneously (in which case collision resolution is needed). In a typical system, a UE is informed, via a broadcast channel (BCH), which access slots the UE can use for a PRACH. Typically, before a PRACH access, downlink (DL) power is measured (e.g., from the BCH) and an initial transmit power is computed from the measurement. In a typical wireless communication system, the preamble does not include the identity of a transmitting UE. If a BS successfully detects the preamble, the BS sends back a random access response message that includes a replica of the preamble, an indication, and resources reserved for uplink (UL) transmission if the indication is positive.
A high-speed PRACH in an LTE compliant wireless communication system employs a relatively complex waveform. An LTE PRACH occupies seventy-two tones at 15 kHz in the frequency-domain and a time-period in the time-domain that is based on a format of the PRACH signal. For example, an LTE PRACH signal that employs format ‘0’ has a time duration of about 0.8 ms (milliseconds). Depending on the formats employed, many PRACH waveforms may be possible. In general, PRACH waveform detection at a serving BS has conventionally been highly complex.
There is an ongoing need for a receiver architecture that is adapted for a multiple user communications system that enables the use of band-width efficient transmissions. More particularly, there is a need for an adequate filtering and decimation structure at the OFDM receiver to recover the data from the various sub-channels it receives.